Electrically driven injectors for injecting fuel are used, for example, within the framework of common-rail systems. In common-rail fuel injection, pressure generation and fuel injection are decoupled. The injection pressure is generated independently of the engine speed and the injected fuel quantity, and is ready in the “rail” for the injection. The start of injection and the injected fuel quantity are calculated in the electronic engine control unit, and are converted by an injector at each engine cylinder via a remote-controlled valve.
Because of their mechanical manufacturing tolerances, such injectors have different fuel-quantity maps. The relationship between the injected fuel quantity, the rail pressure and the triggering time may be understood by a fuel-quantity map. The result may be that, in spite of electrically defined control, each individual injector fills the combustion chamber with different quantities of fuel.
To achieve the lowest possible fuel consumption while adhering to strict exhaust-gas standards, and to achieve very smooth running, the injectors in operation may exhibit only very small tolerances with regard to the injected fuel quantity. It is not possible to adhere to these required low tolerances because of the mechanical manufacturing tolerances. To nevertheless ensure a defined injected fuel quantity for the injectors, after production, the injectors are measured at characteristic operating points for their injected fuel quantity and are arranged in classes. In operation, the specific class must be known to the engine control unit, so that the control may be adapted to the special features of the class, in a manner specific to the injector.
If such a correction of the tolerances by the engine control unit is not possible based on the knowledge of the class, then the special injectors must be reworked mechanically.
There are numerous possibilities for storing the class information on the injector, for example, by various codes, such as by bar code, by resistors on the injector or by plain text on the injector. If the class information is stored on the injector by a code, the information is communicated to the control unit by a code recognition and subsequent programming. If the class information is stored with the aid of resistors on the injectors, the information may be read out automatically by the control unit. However, additional electric lines are necessary. Clear text may be recognized using a camera.
Moreover, it is possible to provide electronic storage possibilities in the injectors, in which, for example, the class information is stored. The control unit is able to read out these values from the injector via an interface and use them in the following operation. However, in this design approach, it is disadvantageous that a separate interface is necessary between the control unit and the injectors.
The injectors may be classified, for instance, by checking the injectors at various tests points with respect to the injected fuel quantity metering. If the measured actual values at all test points lie within a predetermined tolerance window, the injector is evaluated as good. Furthermore, the actual value of one measuring point is used to divide the injectors into three tolerance classes. The tolerance windows of the respective classes each amount to ⅓ of the total tolerance at this test point. Since only an insufficient correlation exists between the test points, a tolerance narrowing at the remaining test points is not possible. Once the injectors have been installed on the engine, the class membership is programmed into the control unit allocated to the engine. The control unit then corrects the injected fuel quantity for the upper and lower class according to a preassigned map. The middle class is not corrected. Because of the poor correlation between the operating points, i.e. the test points, the correction is possible only in the range of the test point used for the classification. In the remaining operating range, at most a slight adaptation of the quantity metering may be carried out on the basis of statistical mean-value shifts between the classes.